Piston actuated pumps are widely known. In certain applications it is necessary to know the position of the pumping piston within the piston cylinder. One method of sensing the position of the pumping piston is through the use of a magnetic reed switch. The magnetic reed switch, however, lacks accuracy and cannot discriminate between two pistons operating within the same cylinder. A mechanically actuated switch could be more accurate and could discriminate between multiple pistons. A mechanical switch, however, presents the problem of sealing between the cylinder interior and the switch. A piston position sensor that incorporated a mechanically actuated switch into a piston and cylinder assembly with a reliable, fluid tight seal between the cylinder and the switch, would be a significant advantage.